2 Answers
2

Super admins do have access to all sites, but they don't show up in "My Sites" by default, which I imagine is where you're looking. They'll want to go to the network admin (/wp-admin/network/, then go to all sites from there, then find the site they want to edit and click "Dashboard" (which appears on hover).

I like to think of "My Sites" almost as favorites, and only add myself to a site when I'll be accessing it frequently, as it reduces the list I have to pick through.

Or For adding your sites on "My Sites" if you've missed them, then you can add them by go to
Network Admin > Dashboard > All Sites > Edit Your Desired Site > Users (Tab) > Add Existing User > Add your desired existing username and role it as "Admin"